1. Field of the Invention
The present invention relates to a flowmeter, and particularly to a flow-rate measurement apparatus that is favorably used as a part of the intake system of an automotive engine and used to measure the amount of intake air.
2. Description of the Related Art
Various types of flow-rate measurement apparatuses to measure the air-mass flow have been known. A type thereof measures the air-mass flow on the basis of the amount of heat radiated from a heat-generating resistor that is controlled so as to generate heat. Another type thereof measures the air-mass flow on the basis of the temperature change of a thermosensitive resistor placed near a heat-generating resistor that is controlled so as to generate heat.
An air filter is provided in the intake duct of a vehicle to remove dust contained in the air that flows into the intake duct. Thereby, the air cleaned by the air filter is introduced into a sub-passage of the flow-rate measurement apparatus.
However, part of the dust—specifically, that of smaller particles—may pass through the air filter. In addition, if the air filter is improperly fitted in its replacement, such an improperly-fitted air filter allows part of the dust to enter the intake duct.
As the amount of depression of the accelerator pedal increases, the dust having entered the intake duct is accelerated together with the fluid up to several tens of meters per second. The dust thus accelerated may even reach the sub-passage of the flow-rate measurement apparatus.
A mass flow measuring element placed in the sub-passage has a very thin portion. The thin portion may be broken if hit by the dust.
In addition, the dust that has entered the intake duct may adhere to the mass flow measuring element of the flow-rate measurement apparatus. If it occurs, the heat-radiation characteristics of the mass flow measuring element may change, and the change in the heat-radiation characteristics may in turn change the output characteristics.
The following technique has been known which provides a structure of protecting the mass flow measuring element from the dust that has entered the intake duct, and of preventing the time degradation of the mass flow measuring element due to the contamination by the dust. Specifically, at a point located immediately after the inlet, the sub-passage that is formed in a spiral shape is branched into two sub-passages—one located in the inner side (inner-circumferential side) and the other located in the outer side (outer-circumferential side). The inner-side sub-passage is provided with a mass flow measuring element whereas the air flowing through the outer-side sub-passage is let out into the intake duct and thus the dust is discharged (see JP translation of PCT International Application No. 2002-506528).